1. Field of the Invention
The present invention relates to methods and materials for conferring disease resistance to plants. More particularly, the invention relates to transgenic plants containing a heterologous nucleic acid which confers disease resistance, particularly against infectious pathogens, such as viruses. The invention further relates to methods and materials for preparation of such transgenic plants.
Infectious diseases of cultivated plants cause substantial reductions in food, forage and fiber throughout the world. Control of these diseases has been based primarily on cultural practices that include removal of infected debris, eradication of weed hosts (herbicide applications), prevention of vector transmission (pesticide applications), indexing for pathogen-free starting material (seed or vegetative propagules) and breeding for disease resistance. Large scale methods for curing plants once they have become infected with viruses do not exist. Thus, the control of diseases is dependent upon methods to prevent or delay the establishment of infection.
Of the above disease control measures, breeding for resistance is generally one of the most economical and practical methods, as it requires no additional labor or expense to the grower. Moreover, controlling diseases with resistance does not require applications of herbicides or pesticides to eliminate weed hosts and insect vectors. Thus, host resistance is one of the most environmentally safe and durable methods for controlling plant diseases. Unfortunately, in many plant-virus systems, resistance is not available and cannot be obtained using traditional plant-breeding strategies. However, recent advances in molecular biology and gene manipulation have proven helpful in integrating new disease resistance factors into plant-virus systems where none existed before.
2. Background Art
The development of transgenic plants has proven to be a valuable strategy for protecting plants from viral diseases. For example, Stubbs, G. et al., U.S. Pat. No. 5,723,750 describe transgenic plants expressing genes encoding wild-type and modified coat proteins of different virus groups. These transgenic plants have been shown to have varying levels of resistance to infection by the corresponding virus. Unfortunately, the expression of heterologous genes encoding coat proteins does not confer broad resistance to viral infections and has no effect on pathogenesis caused by other infectious agents.
An important mechanism of a plant's resistance to a pathogenic infection is the hypersensitive response ("HR"). During the HR, the recognition of a pathogen induces a rapid cell death process that results in the formation of a zone of dead cells (necrosis) around the site of infection. This HR lesion is believed to inhibit further spread of the pathogen and to generate a signal that activates host defense mechanisms and, in many cases, induces long-lasting systemic resistance to a broad spectrum of pathogens (Ross, 1961). (A bibliography is provided at the end of the written description.) Induction of such systemic resistance is termed systemic acquired resistance (SAR) and is accompanied by an increase in the rate of synthesis of several pathogenesis-related (PR) proteins and the accumulation of salicylic acid (SA) (Malamy et al., 1990; Metraux et al., 1990; Ward et al., 1991).
Methods for inducing the HR in infected plants have been described. For example, Lam, E. et al., U.S. Pat. No. 5,629,470, have described a process for providing higher plants with enhanced resistance to pathogenic attack by one or more plant pathogens by transforming cells of the plants with the bacterio-opsin (bO) gene.
In view of the enormous economic impact of infectious diseases on agricultural production, a need continues to exist for transgenic plants having generalized resistance to pathogenic infections.